1. Field of the Invention
The present invention relates to a refrigerant cycle using carbon dioxide as refrigerant, in which a pressure within a radiator exceeds a critical pressure of carbon dioxide.
2. Description of Related Art
JP-B2-7-18602 discloses a refrigerant cycle using carbon dioxide (hereinafter referred to as "CO.sub.2 refrigerant cycle") as refrigerant. In the conventional CO.sub.2 refrigerant cycle, the operation is similar to that of a general refrigerant cycle using flon as refrigerant. That is, as shown by A-B-C-D-A in Mollier chard of FIG. 5, gas CO.sub.2 refrigerant is compressed in a compressor (A-B), and high-temperature high-pressure CO.sub.2 refrigerant in a super-critical state is cooled in a radiator (B-C). The CO.sub.2 refrigerant from the radiator is decompressed in a press-reducing unit (C-D), and is vaporized in an evaporator (D-A). In this case, because CO.sub.2 refrigerant becomes in gas-liquid two-phase when the pressure of CO.sub.2 refrigerant is equal to or less than the saturated liquid pressure of the CO.sub.2 refrigerant, the CO.sub.2 refrigerant is changed from the super-critical state to a gas-liquid two-phase state through a liquid state when CO.sub.2 refrigerant is slowly changed from C state to D state in FIG. 5.
In the super-critical state, CO.sub.2 molecules move similarly to the gas state, while density of CO.sub.2 refrigerant is approximately equal to that of liquid CO.sub.2. However, the critical temperature of CO.sub.2 refrigerant is approximately 31.degree. C. which is lower than the critical temperature (e.g., 112.degree. C. in R12) of flon. Therefore, in the conventional CO.sub.2 refrigerant cycle, the CO.sub.2 refrigerant is not condensed at an outlet (C point) of the radiator in the summer. Further, the state of the CO.sub.2 refrigerant at the outlet of the radiator is determined by a pressure of the CO.sub.2 refrigerant discharged from the compressor and a temperature of the CO.sub.2 refrigerant at the outlet of the radiator, and the temperature of the CO.sub.2 refrigerant at the outlet of radiator is determined by radiating capacity of the radiator and a temperature of outside air. Because the temperature of the outside air is not controlled, the temperature of the CO.sub.2 refrigerant at the outlet of the radiator cannot be controlled actually. Therefore, the state of the CO.sub.2 refrigerant at the outlet of the radiator is controlled by controlling the pressure of the CO.sub.2 refrigerant discharged from the compressor. Thus, to obtain a sufficient cooling capacity (i.e., enthalpy difference) in the summer, it is necessary to increase the pressure of the CO.sub.2 refrigerant at the outlet of the radiator. That is, in the CO.sub.2 refrigerant cycle, it is necessary to increase the compression performance of the compressor, as shown by E-F-G-H-E in FIG. 5.
On the other hand, the compressor is generally lubricated by using a lubricating oil mixed in refrigerant, and the lubricating oil having a high compatibility relative to the refrigerant is generally used to prevent the lubricating oil from staying in an evaporator and a radiator. Further, to supply a sufficient amount of lubricating oil to the compressor, an opening is provided at a liquid refrigerant layer in a gas-liquid separator, and the lubricating oil is introduced into the compressor with the liquid refrigerant. Thus, there are problems that coefficient of performance of the refrigerant cycle is deteriorated and a damage to the compressor is caused.
Further, as described above, in the conventional CO.sub.2 refrigerant cycle, because the operation pressure is high and the amount of the CO.sub.2 refrigerant discharged from the compressor is small, the above-described problems may be readily caused.